The mistress of a Utah doctor accused of killing his wife testified today that their affair was "casual," despite the fact that prosecutors allege she and the doctor exchanged 15 text messages on the day his wife was found lifeless in a bathtub.

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Gypsy Willis said she met Dr. Martin MacNeill, a married father of eight children, online around November 2005, however their relationship did not turn sexual until January 2006, she said.

"We would see each other about a couple times a month. There were months when we didn't see each other. It was a very casual thing," said Willis, who wore a tight blazer over a low-cut camisole while on the stand in the Provo, Utah, courtroom.

Willis testified that MacNeill, 57, helped her financially during nursing school around February 2007.

Prosecutors allege MacNeill drugged and drowned his wife, Michele MacNeill, 50, on April 11, 2007, so he could pursue a relationship with Willis.

"This was a very informal, discreet thing," Willis said of her relationship with MacNeill. "I think he was trying to keep it quiet."

Prosecutors allege the relationship moved full-speed ahead and that phone records indicated the couple exchanged text messages during Michele MacNeill's funeral, which they both attended.

Willis' testimony lasted for about an hour before the judge broke for the weekend. She will return to the stand when the trial resumes on Tuesday.

MacNeill's Youngest Daughter Will Be Allowed to Testify

Judge Derek Pullan said this morning that Ada MacNeill, who was 6 years old when she found her mother unconscious in a bathtub, will be allowed to testify at her father's trial, but with some caveats.

Ada, who is now 12, may have some memories that are "planted or distorted," Pullan said.

Since her mother's death, Ada has been living with her older sister, Alexis Somers, who testified on Thursday that she was asked by investigators to follow up on some details with her Ada.

"There is clear and convincing evidence that some of Ada's memories were planted or distorted by Alexis' repeated interviews," Pullan said.

While no new testimony will be allowed, jurors will be shown a recording of Ada's interview with the Children's Justice Center and then she will take the stand to be cross-examined by her father's defense team.

"There has been a significant lapse of time between then and now," Pullan said. "The degree of her recollection is not stellar."

It was not immediately known when Ada MacNeill would take the stand.

On Thursday, her two older sisters, Alexis Somers and Rachel MacNeill, provided emotional testimony about their father's perceived strange behavior after their mother died.

The day his wife died, Dr. Martin MacNeill insisted on telling Rachel MacNeill how her mother ended up in a bathtub unconscious, even though she said she asked him to not tell her the details.

Rachel MacNeill wiped away tears, and her voice quivered at times, as she testified Thursday at her father's murder trial in Provo, Utah.

"He said that she must have fallen, hit her head," Rachel MacNeill said. "He kept repeating that the autopsy needed to be done. I didn't want to see any of this. My mother just died, and even showing me and talking about [the] autopsy. ... It was horrible. I didn't want to know and I was concerned about my family. I didn't want to hear that."

MacNeill kept his head down for part of his daughter's testimony as she described her close relationship with him while growing up, even calling him her "best friend."

Somers testified that she believed her father was guilty of drugging and drowning her mother as she recovered from plastic surgery. "Ever since the day my mom died, I was concerned that my father killed her. I've been fighting to get justice for this case ever since then," Somers testified.

Much of Somers' testimony focused on her interaction with her youngest sister, Ada.

Despite her opinion that her father is guilty, Somers said she was careful not to taint her sister's recollection of the day their mother died in case she was called to testify at the trial.

Last week, the MacNeill family's next-door neighbor, Kristi Daniels, told the jury that Ada came to her the day of her mother's death and said, "my dad needs some help."

When Daniels entered the home on April 11, 2007, she said she stood in the doorway of the bathroom where Martin MacNeill was with his wife, who was unconscious in the bathtub and whose skin was a "greenish pale" color.

"It all happened so fast," Daniels kept saying on the stand in the courtroom.

Less than two hours later, Michele MacNeill was pronounced dead.

MacNeill's defense lawyers said heart problems were a contributing factor in the death of the mother of eight.